1. Field of the Invention
The present invention relates to molding apparatus in general and in particular to a method and apparatus for encapsulating selected portions of the printed circuit board in a pin grid array.
2. Description of the Prior Art
A pin grid array comprises a printed circuit board, a plurality of pin members which extend from the board and a surface on which is mounted an integrated chip or other electronic component. In a typical embodiment, the electronic components of the chip are connected to electrical pads located on the board surrounding the chip by means of tiny wires. The pads are in turn connected to selected pin members by means of the printed circuitry in or on the surface of the board.
To protect the chip, the interconnecting wires and the printed circuit, some sort of protective cover is provided. The protective cover may take the form of a separate cover member which is attached to the edges of the board. However, separate covers are expensive to manufacture and assemble on the array and obtaining reliable airtight seals with the board is a problem.
To avoid some of the problems associated with using separate covers, various molding techniques have been used for hermetically encapsulating selected portions of the board. For example, in one prior known method, as disclosed in U.S. Pat. No. 4,688,152, the board is clamped between the surface of a lower mold plate and an upper mold plate by means of pin members or ribs in the cavity of the upper mold plate. This method produces indentations in the encapsulating material and exposes portions of the board. It also requires that space be provided on the board to receive the pin members.
In certain applications it is necessary to encapsulate only the upper surface and side edges of the board. In other applications it is necessary to encapsulate all exposed surfaces of the board. In all cases, a good and reliable seal between the encapsulating compound and the board is required to prevent the incursion of air and other foreign material from damaging the electronic components and wiring on the board.
The types of molding compounds used and the thickness of the encapsulating walls may also vary depending on the requirements of the application.